The need for new thermometers that can be used in the presence of strong radio frequency electromagnetic fields has been demonstrated dramatically in recent years (See, for example, C. C. Johnson and A. W. Guy, "Non-ionizing Electromagnetic Wave Effects in Biological Materials and Systems," Proc. of IEEE, 60:692-718, 1972; and T. C. Cetas, "Temperature Measurement in Microwave Diathermy Fields: Principles and Probes," presented at the International Symposium on Cancer Therapy by Hyperthermia and Radiation, April 28-30, 1975) and developments now are appearing to meet this need. The two basic approaches which have been followed have nearly opposite points of view. One method is to start with a well established thermometric technique, in particular, the use of thermistors. The objective, then, is to reduce the electromagnetically induced heating (both electric dipole and magnetic loop currents) through the use of high resistance leads and by minimizing the area enclosed by the circuitry. Examples of these are the MIC devices (see L. E. Larsen et al, "Microwave Decoupled Brain-temperature Transducer," IEEE Trans. on Microwave Theory Tech. MTT-22, 438-444, 1974) and that described by Bowman (see R. R. Bowman, "A Probe for Measuring Temperature in Radio Frequency Heated Material," presented at the International Microwave Power Symposium, Waterloo, Ontario, May 1975, and IEEE Trans. on Microwave Theory Tech., MTT-24, 43-45, 1976). The other approach has been to begin with materials which do not interact with the electromagnetic radiation and then to make a good thermometer, that is, a device which has the appropriate range, has adequate temperature resolution, and can be calibrated. Probe thermometers following this approach are optical devices which use fiber optics to communicate with the sensor and relate its temperature to the intensity of the light reflected from the sensor. Examples of these are a liquid crystal device which is based on the selective reflection of red light (see C. C. Johnson, et al, Discussion paper: Fiberoptic Liquid Crystal Probe for Absorbed Radio-frequency Power Temperature Measurement in Tissue During Irradiation, (in) Biologic Effects of Non-ionizing Radiation, Ann. N.Y. Acad. Sci., February 1975, 247:527-531; and T. C. Rozzell et al, "A Non-perturbing Temperature Sensor for Measurements in Electromagnetic Fields," J. of Microwave Power, 9:241-249, 1974) and the birefringent crystal sensor which is the subject of this patent application. This latter development by T. C. Cetas appears in a paper, "A Birefringent Crystal Optical Thermometer for Measurements of Electromagnetically Induced Heating," which was presented at the International Union of Radio Science (URSI), Boulder, Colorado, October 1975. Two inherent advantages of the birefringent crystal sensor are the stability of the sensor itself (a solid single crystal in contrast to a semi-ordered liquid) and the existence of a simple, physically based, expression to characterize the thermometer calibration.
The present invention consists of three configurations of a probe-type thermometer utilizing a birefringent crystal at the tip of the probe and these variations have been generally described above. As noted above, the first configuration is an assembly of a probe thermometer made by mounting a polarizing disc, a birefringent crystal (anisotropic) with temperature dependent indices of refraction and a mirror at the end of a bundle of optical fibers. Half of the fibers in the bundle lead to a light source such as a light emitting diode, the other half lead to a photodetector, such as a photodiode. Appropriate electronics energize the light emitting diode and amplify and measure the signal received by the photodetector. One difficulty with the device is that a slow drift may occur in the calibration as a result of drifting characteristics of the light source and light detector. The present application discusses another method of assembly which eliminates the effects of drifting electro-optic components.
The second basic configuration is sketched in FIG. 3. Instead of two fiber optic bundles going to the sensor, only one leads from a beam splitter to the sensor. This sensor arrangement is simpler and is easier to construct and to miniaturize.
A third configuration substitutes a second reference photodetector circuit for a chopper and wheel.